This invention relates to a vacuum electronic device such as a fluorescent display device or the like and an envelope therefor, and more particularly to a structure for evacuating an envelope to keep it at a high vacuum.
In general, a conventional fluorescent display device includes an envelope kept at a high vacuum, in which a cathode for emitting electrons is arranged. Also, the envelope is provided on an inner surface thereof with a display section including phosphor layers and anode conductors and arranged opposite to the cathode. Also, the envelope is provided therein with control electrodes so as to be positioned between the cathode and the display section, as required. The fluorescent display device thus constructed causes electrons emitted from the cathode to be selectively impinged on the phosphor layers, to thereby provide a desired luminous display.
The cathode comprises a filamentary cathode adapted to be electrically heated to emit thermions, a cold cathode such as a field emission cathode, or the like.
The envelope is formed of a substrate on which the above-described display section is provided, a cover plate arranged oppositely to the substrate, and side plates constituting a side wall of the envelope. The substrate or cover plate is provided with an evacuation tube or an evacuation hole, through which the envelope is evacuated. Thereafter, the evacuation tube is sealed or the evacuation hole is closed with a lid member or the like, resulting in the envelope being kept at a high vacuum. Alternatively, the side wall of the envelope is provided with an evacuation tube, through which the envelope is evacuated, followed by closing of the evacuation tube.
Unfortunately, the above-described conventional envelope for the fluorescent display device meets with several disadvantages.
More particularly, in order that sealing of the envelope after evacuation of the envelope through the substrate or cover plate opposite to the substrate permits the envelope to be kept at a high vacuum, in is required than the substrate or cover plane is formed with such an evacuation hole as described above. Unfortunately, this leads to problems due to dusts, contamination, cracks of the substrate or the like occurring during formation of the evacuation hole. In particular, when it is required to subject both substrate end cover plate to fine processing as seen in the fluorescent display device having a field emission cathode incorporated therein, dusts or contamination occurring during formation of the evacuation hole often adversely affects the fine processing. Also, the evacuation hole per se adversely affects an exposure step or the like in the fine processing, resulting in the fine processing being failed.
Another disadvantage of the conventional fluorescent display device is that arrangement of several evacuation holes at the substrate causes cracks to occur in the substrate and the like.